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Recovering refrigerant?

When an automotive ac system has to be opened for repair the refrigerant (was R12, since 1995 it's R134) is recovered and later reinstalled. Following repair the system is vacuumed to 29.9" mercury (I think that's about 500 microns to you guys). If the system holds vacuum it is recharged by weight to a precise spec. Most systems nowadays hold 1 to 2 lbs. The high and low side pressures are only checked to ensure the mechanical components are functioning properly. As a matter of course the amount of refrigerant is never adjusted up or down.
I understand that you guys look at superheat, supercool, and pressures and adjust refrigerant accordingly. Is this because there are many variables we don't deal with in the automotive ac biz? Like line length, mismatched condensers and air handlers? Or what?
Also, do you recover the refrigerant when you open a system? I understand some or all of it can be held in the condenser. How do you get it there? Do you seat the valve on one side and pump down the lines and the evaporator and pump it into the condenser?

(As an aside, if you think you've dealt with some hackers in your trade you wouldn't believe what we see in the automotive trade. How about filling the system with propane just for starters? Our Wall of Shame would have you figuring out whether you should be laughing or crying!)

I know less about automotive AC than just about anything. But it's my understanding automotive units have evaporator regulators to control refrigerant flow because of the dramatic differences in airflow (selectable by the driver or his/her assistant). So much like our mini-splits where a single refrigerant charge covers all operational variations, we do as you've described and weigh in the charge. As far as recovery is concerned, it all depends on the operation to be done. If the operation is limited to opening the system external to the condenser, then yes, we pump down and since the service ports are accessed to the coil and lines external to the condenser, we can evacuate to well below 29.9 in hg into the micron range and then down to 500 microns, which we consider to be a dry system, then we release the refrigerant trapped in the condenser. For work on the condenser side, we recover all refrigerant and normally recharge by weighing in, then setting the final charge by superheat and/or subcooling as appropriate to the metering device in use.

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I know less about automotive AC than just about anything. But it's my understanding automotive units have evaporator regulators to control refrigerant flow because of the dramatic differences in airflow (selectable by the driver or his/her assistant). So much like our mini-splits where a single refrigerant charge covers all operational variations, we do as you've described and weigh in the charge. As far as recovery is concerned, it all depends on the operation to be done. If the operation is limited to opening the system external to the condenser, then yes, we pump down and since the service ports are accessed to the coil and lines external to the condenser, we can evacuate to well below 29.9 in hg into the micron range and then down to 500 microns, which we consider to be a dry system, then we release the refrigerant trapped in the condenser. For work on the condenser side, we recover all refrigerant and normally recharge by weighing in, then setting the final charge by superheat and/or subcooling as appropriate to the metering device in use.

Very interesting. Automotive systems all have a compressor, most are turned on and off by a low pressure switch and/or a thermostatic switch inserted into the evaporator fin area. Most every car with an on/off compressor(usually called a cycling clutch compressor) have both devices. As you know, temperature and pressure are related and therefore either device can be the main source of a temperature regulation signal.

In the past few years wehave begun to see variable displacement compressors which have no clutch! They spin whenever the engine is running. However, they have swash plate "crankshafts" which vary the piston travel, from nothing to full travel depending on load.
The refrigerant is squeezed down to a hot high pressure gas which then travels through the condenser where it is liquified and then to a receiver/drier which is very similar to yours. (When you look through the grille of a car what you are seeing isn't the radiator, it's the condenser.)

Before entering the evaporator the liquid refrigerant has to pass through a valve. Many years ago every system had a TXV, almost identical to yours except slightly smaller. Their usage is becoming more rare and we now mostly have "H-block valves" and orifice tubes. The H-blocks look very different from a TXV but they work very similarly, the liquid refrigerant is throttled by a needle valve controlled by a chamber which expands and contracts with temperature. An orifice tube has a fixed 'hole' which meters the refrigerant. Obviously this has to be set to maximum acceptable flow. It depends on the cycling compressor and temperature blend doors under the dash to arrive at a happy passenger.
You'd be right if you said this is not a very efficient way to do it. Basically the AC compressor is running at Max Cool whenever it's on.\

In the automotive ac trade we do very few -OK, none- new installs. We are repairmen. Most system failures are due to loss of refrigerant caused by bad o-rings, there are 8 to 12 of them in every system, or perforated condensers, or corroded aluminum lines. We don't use copper. The lines are all aluminum tubing and hi-pressure rubber hoses. Pressures range from 20-25 on the vapor side and around 175-200 psi on the liquid side. High side pressures can eaily hit over 400 psi due to fan failure before the hi-press safety switch cuts out the compressor.

Compressor replacement is fairly common, and isn't too bad a job. The hardest job is evap replacement due to the fact that in most cases you have to remove the entire dashboard and steering wheel to get at the HVAC box which then has to be disassembled to access the evap. It's an all day job in most cases.

I guess both our professions have their good and bad points, the good days and the not so good days. One real similarity is that if customers took better care of their systems they'd last a lot longer. Pay a little now, or a lot later, your choice.